John the Ripper

http://www.openwall.com/john/
John the Ripper is a fast password cracker, currently available for many flavors of *nix (11 are officially supported, not counting different architectures) and other platforms. Its primary purpose is to detect weak Unix passwords. It supports several crypt(3) password hash types which are most commonly found on various *nix flavors. Several other hash types are added with contributed patches.

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